Method for limiting stresses in elongated elastic structures



March 10, 1953 J. F. KENDRICK METHOD FOR LIMITING STRESSES IN ELONGATED ELASTIC STRUCTURES Filed Aug. 29, 1949 5 STROKES PER MINUTE RATIO )OF OF NATURAL VlBRATlON (T) DlVlDED sv PERlOD or RECIP- 4000--p-5000 WEIGHT OF TOOLS POUNDS 3 Sheets-Sheet 1 Jaiznl'Kendrick mrneg March 10, 1953 V KENDR|K 2,631,011

METHOD FOR LIMITING STRESSES m ELONGATED ELASTIC STRUCTURES Filed Aug. 29, 1949 3 Sheets- Sheet 2 [Line J'okn FKendriclt' 3 Sheets-Sfieet a J. F. KENDRICK March-10, 1953 METHOD FOR LIMITING STRESSES IN ELONGATED ELASTIC STRUCTURES Filed Aug. 29, 1949 Joan filfndrz'ck Patented er. 1:), i953 UNITE METHOD FOR LIMITING STRESSES IN ELONGATED ELASTIC STRUCTURES John F. Kendrick, Columbus, Ohio Application August 29, 1949, Serial No. 112,858

18 Claims.

This application is a continuation-in-part of Serial No. 585,871,.filed March 31, 1945, now abandoned, Method for Limiting Stresses in Elongated Elastic Structures. Also pending is continuation-in-part application Serial No; 119,236, filed October 3, 1949, Method for Limiting Stresses in Cable-Tool Drilling Lines. Divisional applications Serial No. 115,282, filed September 12, 1949, Cable-Tool Drill with Electrical Variable Crank and Serial No. 116,117, filed September 16, 1949, Cable-Tool Drill with Mechanical Variable Crank, which were currently pending, have been abancloned.

This invention relates to the method and apparatus for the safe operation of elongated elastic metallic structures, such as the cable tool drill, through a range of velocities, or S. P. M., requiring passing back and forth through zones of vibrational resonance. More specifically, it relates to apparatus adapted for changing or varying the length of the stroke of a crank operated reciprocating machine at the will of the operator and while the same is in operation. Also, it pertains to means for changing the effective radius of a crank, thereby varying the length of the stroke, for example, of an associated cable tool drill for sucker-rod pump, as desired by the operator, while such apparatus is in operation.

A cable tool drill consists of a bit, as part of a string of tools, attached to a drilling cable, which formerly was a manila rope but is now most often a wire line. Two forms of crank operated machines, the walking beam (Fig. 1) and spudder (Fig. 2), are commonly used to reciprocate the drilling cable and tools so that the im act of the bit against the formation will drill a hole or well bore. It will be observed that as the depth of the well increases, the length of the drilling cable is increased in proportion.

When the well is quite shallow, there is relatively little elasticity in the drilling cable and the mechanism conforms approximately to the physical laws of ri id bodies. A depth is reached eventually, depending on the weight of the string of tools and the size and type of the drilling cable, where the elasticity has increased to the point at which the bit reaction must be determined by the laws of elasticity. Either form of cable tool drill is a modification of a vibrating system with one degree of freedom, with forced vibrations and with viscous damping, the mathematics of which is fully developed in any one of a number of standard reference books, such as pages 1-113, particularly pages 38-51 of Vibration Problems in Engineering by Timoshenko, published by D. Van Nostrand Co. Each departs slightly from a strict conformance to the theory due to the impact of the bit with the f rmation. This reduces the length of stroke of the hit approximately in proportion to the magnitude of the coeificient of restitution, which depends on such factors as the hardness of the formation, which, in turn, determines the depth of penetration of the bit,

The general characteristic of the stroke of the tools is shown in Fig. 3. The vertical scale is the stroke of the tools expressed as multiples of the surface stroke. The elastic drilling cable has a natural or fundamental frequency, and the operation of the walking beam or spudder arm is a forced vibration with a fixed frequency at any given number of strokes per minute. The horizontal scale is the ratio of the period of the natural frequency of the drilling cable to the period of the vibration of the walking beam or spudder arm, and this scale can be expressed in strokes per minute for any fixed set of conditions, by methods well known to those familiar with vibrational mechanics. Whenever there are two or more vibrations working together, we encounter a critical velocity or resonance at which the frequencies are equal or nearly equal, which is called the peak of vibrational resonance in this specification, where the amplitude or half-stroke of the resultant vibration is magnified greatly. Adjacent such peaks are zones of destructive vibrational reactions. It will be observed in Fig. 3, therefore, that at relatively low strokes per minute of the drilling mechanism, the stroke of the bit is equal to the surface stroke; at the critical velocity of the resonance point, the stroke of the bit is magnified greatly, and at high speeds the stroke of the bit becomes less than the surface stroke.

The strokes per minute at which resonance occurs decrease as the depth of the well hole increases as shown in Fig. 3a. While the presence of drilling water atthe bottom of the hole introduces a slight amount of viscous damping and the coefiicient of restitution further reduces the violence of the bit reaction at resonance, experience has shown that the reaction is such as to throw slack into the drilling cable, which up-- sets the rhythm of the drilling motion and frequently causes breakage. It is, therefore, customary to operate the cable tool drill at a speed well below resonance, generally at a ratio of the periods of the two vibrations of approximately 0.6. As the drilling speed is a function of the impacts per minute, other factors remaining the same, improved performance will result if the cable tool drill is operated above resonance, beyond the depth where the elasticity of the drilling cable has reduced the usual drilling speed to a point where it is no longer economical.

In order to operate the cable tool drill above resonance, it is necessary to pass through the critical velocity or resonance range safely to attain a speed at which the stroke of the .bit is less than what it is below resonance to compensate for the increased velocity. This can be done by increasing the strokes per minute, or motion as it is called by drillers, rapidly so as to pass through the resonance stage fast enough to prevent the stroke of the bit from becoming excessive. However, it is impossible to guard against the bit being operated for a few strokes with an impact sufiiciently large to seriously injure its cutting edge. Also, the weight of the drilling cable is such, on the deeper wells, that the cable may be damaged before the drilling motion settles down.

Again some degree of safety can be secured by setting the tools on bottom and picking them up after the desired speed be done relatively easily with the spudder type of driller, but is difficult with the walking beam type, unless a hydraulic "temper screw" is available. Also, the weight of the cable increases, as drilling proceeds, so that this protection becomes less as the weight of the cable approaches and exceeds the weight of the tools. A rapid increase in the strokes per minute in combination with the setting of the tools on bottom will aff rd a greater degree of protection, but all such expedients lack the precision control that results in complete safety.

The sucker rod pump is generally operated by slight modifications of the walking beam of Fig. 1. The sucker rods are joined together by threaded joints and the pump is attached to the lower end of the string of rods. In some fields, the sucker rods are replaced by a wire line. It departs from the fundamental theory of a vibrating system with one degree of freedom in that the sucker rods are relieved of the mass of the fluid on the down stroke. However, experience has shown that the reaction at resonance is too severe to permit safe operation. Passing through the critical velocity rapidly is the only safety precaution available, as it is impractical to relieve the sucker rods of the weight of the fluid. While the advantages of operating the sucker rod pump above resonance are somewhat coniectural, it is possible that operation at such speeds would relieve the sucker rods of many very destruction secondary vibrations.

In all such crank operated reciprocating machines, intended for operation over a wide range of speeds, the desired safety can be obtained by the precision control resulting from changing the length of stroke, at will, while the machine is in operation. The preferred method of doing this is to slidably mount the wrist pin in the crank, so that when it is centered with the drive shaft, the stroke will be zero, but suiilcient movement radially will be provided to give the longest stroke desired. The preferred method of operation is to control the position of the wrist pin by means of a lead screw operated by an e ectric motor intermittently and reversibly, controlled through slip rings by a push button, or other type of switch, within reach of the operator. Mechanical and hydraulic modifications are hereinafter disclosed as alternates.

With such a crank mechanism, it will be possible to reduce or eliminate the throw of the crank as the machine is started up, so as to limit or eliminate the excessive stresses or destructive reactions, as the zone of vibrational resonance is traversed. Once the machine is operating at a speed above that equivalent to the range in the zone of vibrational resonance, the throw of the crank can be increased at the will of the operator, thereby applying the load to the component parts of the machine gradually and under satisfactory control. Once normal operation is established, it will be necessary only to reduce or eliminate the amplitude of the reciprocation or throw of the crank, when the frequency or S. P. M. are above the peak of vibrational resonance and then reduce the frequency or S. P. M. to traverse safely the zones of destructive reactions adiacent the peaks of resois reached. This can.

nance. Starting up again. it will ordinarily be unnecessary to further adjust the amplitude of reciprocation or stroke of the crank, until the frequency or S. P. M. have been increased to a point above the peak of vibrational resonance, whereupomthe amplitude of the reciprocation or throw of the crank will be increased to give the desired reciprocation.

It is an object of this invention to provide a new and economically important method of operating elongated elastic structures, such as the cable tool drill. It is a further object of this invention to provide a means for varying the stroke of a crank operated reciprocating machine from zero to a maximum at the will of the operator, while the machine is in operation. It is another object of this invention to make it possible to bring the other parts of such machines up to the desired speed before the load is applied by increasing the length of the stroke from zero and to assure a high degree of safety in handling large elastic masses through a wide range of speeds by the precision control resulting from increasing or decreasing the length of th stroke gradually. It is a further object of this invention to so control the impact of the bit of the cable tool drill by changing the length of the stroke gradually to afford it the maximum protection against breakage or unnecessary wear. It is another object of this invention to regulate the impact of the bit to conform to the hardness of the formation, while still maintaining the highest practical number of blows per minute.

For a further understanding of the nature of the invention and the detailed features of construction thereof, as well as additional objects and advantages, reference is to be had to the following description and the accompanying drawings, wherein:

Fig. 1 is a diagrammatic side elevational view of a standard or walking beam type of cable tool drill;

Fig. 2 is a similar view of the spudder type of cable tool drill:

Fig. 3 is a graph diagram showing the theoretical length of the stroke of the suspended weight in a vibrating system with one degree of freedom;

Fig. 3a is a graph diagram showing how the resonance peak occurs at a lower number of S. P. M. as the depth of the well increases;

Fig. 4 is a longitudinal sectional view of a crank having an adjustable wrist pin and showing the electrically controlled embodiment of this invention;

Fig. 5 is a front'elevational view thereof;

Fig. 6 is a sectional view of a crank and wrist pin and showing a mechanically controlled modification of this invention;

Fig. 7 is a front elevational view of the brake sleeve;

Fig. 7a is a vertical longitudinal sectional view taken through the brake sleeve;

Fig. 7b is a rear elevational view of the cross head operating sleeve;

Fig. 7c is a vertical longitudinal sectional view taken through the cross head operating sleeve and disclosing its turning ring applied thereto;

Fig. 7d is a vertical transverse sectional view on the line X--X of Fig. '70;

Fig. 8 is a sectional view of a crank and wrist pin showing an hydraulically controlled modificaion.

Referring to Fig. 1, showing the principal parts of the Walk g beam type of cable tool drill, the

5 numeral I is the band wheel driven by belt 2 from a motor or engine (not shown). A crank 3 is mounted'on the band wheel shaft and carries a wrist pin 4. The wrist pin is connected to a pitman 5 which at its upper end is connected to one end of a walkin beam 6, the latter being supported for oscillation by the usual Samson post. From the opposite end of the walking beam are .suspended a temper screw 1 and a clamp 8. which grips a drilling cable 9. The bit and other parts of the string of tools (not shown) is suspended from the lower end of the drilling cable. In operation, the crank imparts an oscillating motion to the beam which in turn reciprocates the drilling cable. Thus, the bit is able to strike the formation with impact, which eifects the drilling. A modification of this mechanism is commonly used to operate a sucker rod pump. My invention is incorporated in the crank 3 and its associated wrist pin 4.

In Fig. 2, which shows the principal parts of the spudder type of cable tool drill, I is an engine which drives the crank 3a through a belt 2a and -wheel la, mounted on a shaft common to the crank 3a. The crank drives the wrist pin ta, which causes the upper end of the pitman 5a to oscillate the spudder arm H, which imparts a reciprocating motion to the drilling cable 9a. This causes the bit (not shown) suspended from the lower end of the drilling cable to strike the formation with impact, which eifects the drilling.

In operation, a string of sucker rods or a drilling cable usually attains such length as to acquire sufiicient elasticity to conform closely to the principal characteristics of a vibrating system, having one degree of freedom with forced vibrations and viscous damping. The Suckerrod Pump as a Problem in Elasticity by John F. Kendrick and Paul D. Cornelius, published on pages to 31 inclusive, of volume 123, Transactions of the American Institute of Mining and Metallurgical Engineers, 1937, conveniently brings together the principal formulas for such a system. Fig. 3 is the graphical solution of Formula 1, page 17, preceded by plus and minus, with 1 substituted for 12 to give a dimensionless factor. thus:

2 T?) 1? where d3=half the subsurface stroke, ins.,

T=period of natural vibration, sec.,

T1=period of forced vibration of reciprocating mechanism, sec., =60/S. P. M..

u=damping factor.

The upper half of this group of curves only is most generally reproduced, and it is the amplitude or half stroke of the sub-surface vibration. Prefixing plus and minus results in the mirror image of the upper group of curves, so that an upper with its corresponding lower curve gives the boundary of the displacement or the full length of the resultant sub-surface vibration or stroke in multiples of the surface stroke, as given on the vertical scale. The different curves, marked 0.3, 0.5 etc., are the boundary of the resultant stroke under different friction factors.

A string of sucker rods or a drilling cable are elastic bodies, which, when excited, as by the reciprocating motion at the surface, will vibrate with a natural vibration which has a definite pcriod in seconds per cycle, which is the reciprocal of the frequency in cycles persecond. The period T in seconds, per cycle, of the natural vibration, is equal to 0.3195 times the square root of the static deflection, which will be explained later. The period of the forced vibration, or the surface reciprocation, T1 in seconds per cycle, is, naturally, equal to 60 divided by the S. P. M. The horizontal scale of Fig. 3 is the ratio R of the natural vibration T divided by the period of the forced vibration T1. The horizontal scale may be converted to S. P. M. N by multiplying R by 60 and dividing by the period of the natural vibration T, as given by Formula 2, page 20,

of Kendrick et al., thus:

where N=strokes per minute of the reciprocating mechanism, R=ratio T/Tl, 0.2, 0.3, etc.,

and

T=period of the natural vibration, sec.

The static deflection (is is the distance the elongated elastic structure, drillingcable or sucker rods, as the case may be, will stretch when subjected to a load equal to one third their own weight plus the weight of the tools or fluid suspended at its lower end, as determined by Hookes Law (Formula 5, pag 25, Kendrick et a1.), as is explained in detail, starting at page 25 of Kendrick et al., thus:

, 'AE where e=elongation, in., P=force producing stretch, lbs., L=length of spring, in., A=area, sq. in., E=modulus of elasticity.

In the case of the cable tool drill, the aging of the drilling cable is evidenced by an increase in the magnitude of Youngs modulus E. The location of the peak of vibrational resonance should therefore be computed for a maximum and minimum modulus, using the manufacturers value for the minimum modulus and the average of this and thirty million, the modulus for steel, as the maximum modulus. This results in two loci of the peak, as shown in Fig. 3a. Fig. 3a is plotted by computing the periods of the natural vibration of the system, using the minimum and maximum moduli, and the physical data for the conditions being analyzed, as indicated at the right edge for different depths, and dividing 60 by the periods T, to get S. P. M., at vibrational resonance or when R is equal to l. The modulus of a string of sucker-rods does not change significantly with age, so Fig. 3a for a sucker-rod pump would have just one curve based on the modulus for steel of thirty millions.

In the operation of the sucker-rod pump above the resonance peak. suflicient volumetric displacement of the pump plunger within the maximum allowable rod stress is all that is required. The length of stroke and the S. P. M. for operation of the pump below the peak of vibrational resonance are well established in any given field. The engineer would compute the S. P. M. for equivalent plunger displacement above the resonance peak, as indicated above, reducing the sur- V and radial bearings l1 and i8.

face stroke as required. He would then check the rod stress with dynamometers well known in the art, and give the pumper, who actually opcrates the pumps, a table of lengths of surface strokes and S. P. M. In the operation of the drill, the point of operation would be determined as above. Actual drilling involves the additional problems of controlling the impact and the rate of feed of the bit. A solution to these problems is disclosed in my co-pending appiication'serial No. 590,498, filed April '26, 1945, Drilling Motion Indicator for Cable-Tool Drill. This application issued January 29, 1952, as the following patents: No. 2,584,026, Apparatus for Drilling Indicators; No. 2,584,027, Drilling Cable with Insulated Conductor; No. 2,584,028, Impact Switch. In either case, operation below the resonance peak is at a frequency or S. P. M. which are less than when operating above the peak, and with an amphtude of vibration or reciprocation or length of stroke which is greater than when operating above.

As stated above, the corresponding pairs of curves of Fig. 3, marked 0.3, :5 etc., are for different damping or friction factors. Viscous damping or friction has very little effect at very low or very high speeds, but does lower the peak which occurs at resonance, which is the critical velocity, where the periods of the two vibrations are equal or nearly equal. In the cable tool drill and the sucker rod pump, the viscous damping is caused by the fluid in the well, and while it is greater in the pump than in the drill, it is relatively small in either case. While the impact of the bit with the formation further lowers the peak in the cable tool drill, the reaction at resonance is too violent in either case to permit safe operation at the corresponding speed. It is customary to operate such mechanisms at a speed below resonance and it is the purpose of this invention to make it safe to operate at some speed above resonance, and adjust the surface stroke for the most eillcient operation at such a speed.

In Fig. 4, showing the preferred embodiment of this invention, 3 is the crank keyed to a shaft i2, which is supported by a jack post bearing l3. The wrist pin 4 is slidably mounted in the crank by means of inner and outer ways l4 and IS. The wrist pin is moved along the ways from a position in axial alignment with the shaft, at

which point there will be no eccentricity, to a. 0

point close to the outer end of the crank, resulting in the maximum eccentricity or stroke, by means of the rotatable lead screw IS, the latter being supported axially by means of thrust The inner end of the lead screw is connected through suitable bearing I! to an electric motor mounted on the crank, so that motor, transmission and lead screw all rotate with the crank around the center axis of the shaft. The control wires 2i, of the motor, are connected to sliding brushes 22 which are supported by a bracket 23 attached to the crank 3. The sliding brushes 22 contact collector rings 24, which are stationarily mounted by means of bracket to the jack bearing post i3. These collector rings 24 are connected to a source of power, not shown, through the magnetic contactor 25, controlled at will by the operator by means of a push button switch 21, which are catalog items.

Normally, the motor is not running but when one push button is depressed, the motor will revolve in one direction and when the push button is-released, the motor will stop running,

when the other push button is depressed, the motor revolves in the opposite direction. In this way, the lead screw may be turned in one direction or the opposite direction advancing or retracting the wrist pin, thereby increasing or decreasing the effective stroke of the crank at the will of the operator.

It is preferable that the lead screw be selflocking by the use of a small pitch. While spur gearing is shown, this may be replaced by bevel gearing, a worm and gear orany combination necessary to meet a range of conditions, as the axis of the motor may be put at right angles to the axis of the crank. Also the number of sliding brushes and collector rings will vary according to the characteristics of the electrical current to be used.

In Fig. 6, showing the mechanically operated modification of this invention, 3 is the crank keyed to a shaft l2, which is supported by a jack post bearing i3. The wrist pin 4 is slidably mounted in the crank by means of ways i4 and I5. The wrist pin is moved along the ways from a position in axial alignment with the shaft, at which point there will be no eccentricity, to a point close to the outer end of the crank, resulting in the maximum eccentricity or stroke, by means of the lead screw 5, supported axially by means of thrust and radial bearings i1 and it. as in Fi 4.

Two gears 23 and 28 are keyed to an extension of the centrally located end of the lead screw. Gear 28 engaged directly a mating gear 30, which is slidably mounted on the hub of the crank in axial alignment with shaft i2. Gear 29 drives an associated gear 32, slidably mounted on the hub of the crank in axial alignment with shaft i2, through an idler pinion 34. Normally, gears 30 and 32 rotate with the crank and there is no rotative movement of gears 28 and 29 about the axis of the lead screw, so there is no movement of the lead screw l6 and wrist pin 4 is held stationary. However, gear 30 is provided with a brake drum 3| and gear 32 is provided with a brake drum 33. The construction of the double acting radial brake sleeve 35 will be more easily understood by referring to Fig. '7. This consists of a sleeve slidingly mounted in longitudinal alignment with the crank hub, provided at the end thereof a hacent to the crank 3 with a radial slotted flange 36. Crossheads 36a are siidingly mounted in these radial slots, which have segmental brake bands 31 and 38 on either side and brake band 31 may engage inwardly the brake drum 3i of gear 30 and brake band 38 may engage by its outward movement the brake drum 33 of gear 32. Each crosshead is provided with a slanting operating slot 39.

The other or bearing end of the radial brake sleeve is provided with a threadedly attached flange 40, which has a plurality of tapped holes 4|, circumferentially arranged. Coupling pins 42 are screwed into these holes which engage flexibly a coupling flange 43 (Fig. 6) which is stationariiy mounted to the bearing post i3 by means of bracket 44. Therefore, when brake band 3! is engaged inwardly with brake drum 3! of gear 30, gear 30 is held stationary by the brake sleeve 35 which is held against rotation by its engagement through pins 42 to coupling flange 43, and which is attached to the bearing post i3 by bracket 44.

The rotation of the crank 3 causes gear 28 to run around the outer circumference of gear 30, which rotates the lead screw it, moving wrist heads outwardly, disengages the brake drum 3| of gear 30, and-when thecrossheads are in the midway or neutral position, neither gear 30 or 32 will move in relation to the crank hub. When the crossheads are moved outwardly to their extreme positions, brake band 38 engages outwardly the brake drum 33 of gear 32, which is then held stationary as a result of its engagement with coupling flange 43 through the brake sleeve and coupling pins 42. Idler 34 is, therefore, constrained to run around the circumference of gear 32, due to the rotation of the crank, which causes ear 29 to revolve about the axis of the lead screw.

The direction of rotation of the lead screw is opposite to what it is when the drive is through gear 28, due to the operation of idler 34. Therefore, wrist pin 4 is moved along the crank 3 in a direction opposite to what it is when the drive is through gear 28. Meanwhile gear 30 is free to turn on the crank hub idly because its brake drum 3| is disengaged. While four brake crossheads are shown in Fig. 7, it is obvious that any convenient number can be used.

The radial sliding of brake crossheads is controlled by the brake-operating sleeve 45a, which fits over the brake sleeve of Fig.- 7. The crank end of this sleeve is providedwith a plurality of radial spokes 45, which have spirally shaped flanges 46, directed outwardly from a plane at right angles to the axis of the sleeve. These engage slidingly mating slots 39 in the crossheads 36a. Therefore, when the brake-operating sleeve is oscillated about its longitudinal axis, the crossheads are moved in or out radially, thereby alternately engaging the brake bands 31 and 38 with their respective brake drums 3| and 33. The oscillation of the brake-operating sleeve is effected by the machine operator through a system of levers or a system of cords and pulleys (not shown) connected to lever arm 41 which protrudes radially from collar H, which is keyed to the brake-operating sleeve at 12 and held in position relative to the sleeve by set screws 13.

The hydraulically operated modification of this invention is shown in Fig. 8. The crank 3 is fastened to the flange 48, which is keyed to shaft l2, and which is supported by jack post bearing l3. The wrist pin 4 is slidably mounted in the crank by means of ways 14 and I5. A hydraulic cylinder 49 is fastened in axial alignment to the crank and its piston 59 is'connected to the wrist pin 4 by means of piston rod Hydraulic port 52 is placed in one cylinder head of the hydraulic cylinder and port 53 is placed in the opposite cylinder head. Hydraulic cylinder, piston and piston rod are free to revolve with the crank.

Two grooves 53' and 54 are machined circumferentially around the hub of the crank, eccentrically to the center of the crank. Conduits 55 and 56 are drilled radially in the hub, between the crank and the circumferential grooves 53' and 54, and are connected with circumferential grooves 53' and 56 by means of conduits 51 and 58, drilled longitudinally in the crank hub, so that conduit 51 intersects circumferential port 53' and conduit 55, while conduit 58 intersectscircumferential port 54 and conduit 56. The end of conduit 51 adjacent shaft flange 48 is closed by means of plug 59, while the end of conduit 58 adjacent shaft flange 48 is closed by means of plug 60. Conduit 51 is connected to one end of the hydraulic cylinder 49 by means of tubing 6|, while conduit 58 is connected to the other end of the hydraulic cylinder 49 by means of tubing 62. I

The circumferential ports 61 and 58 are enclosed by a recessed collar 63, stationarily mounted in longitudinal alignment with the crank hub and shaft l2 to the jack bearing pedestal I3 by means of bracket 64. The recess of collar 63 is closed by cover plate 65; The

circumferential grooves are separated hydraulically by means of packing 66 positioned in the; recess of the collar 63 and the separators 61.

Provided, but not shown in Fig. 8, are a reservoir, pressure pump with suitable engine or motor and regulators; and a control valve located within reach of the operator. The reservoir is connected to the suction inlet of the pump. The pump discharge is connected to the control valve, with the usual pressure controlled by-pass to the suction side or reservoir. The proper element of the control valve is connected to the reservoir or the pump suction through a back pressure valve. The remaining two positions of the control valve are connected to pipes 68 and 69 of Fig. 8 so that when the control valve is in one position hydraulic pressure is admitted to one end of cylinder 49, while the other end of cylinder 49 is connected to the reservoir through the control valve and the back pressure valve. A reversal of the control valve reverses the connections, whileholding the control valve in the neutral position maintains hydrostatic pressure on both sides of piston 59. Therefore, when the control valve is positioned to admit pressure fluid to pipe 68, the fluid flows through conduits 54, 58 and 56, through pipe 62 and port 52 to the upper side of the piston.

At the same'time, the pressure 'is reduced on the lower side of piston 59 by the fluid passing through port 53, pipe 6|, conduits 55, 51 and 53 to pipe 69 from which it goes through the control valve 68a to the pump intake or reservoir through the back pressure valve. The wrist pin 4 therefore moves in toward the center of the crank. This movement ceases when the control valve 68a is put in a neutral position.

Reversing the control valve admits the high pressure fluid to pipe 69, conduits 53, 51 and 55 to pipe 6|, port 53 to the bottom of piston 50. At the same time, the fluid on the other side of piston 50 passes through port 52, pipe 62. conduits 56, 58 and 54 to pipe 68. From pipe 68, it flows through the control valve 68a to the reservoir or pump intake through the back pressure valve. The wrist pin is thereby moved away from the center of the crank and the stroke is lengthened. Putting the control valve in its neutral position stops the movement of the wrist pm.

The position of the wrist pin of the electrically operated structure of Fig. 4 and the hydraulically operated modification of Fig. 8 may be changed with the machine at rest or in motion at the will of the operator, while the position of the wrist pin of the mechanically operated modification of Fig. 6 can be changed only while the machine is in operation.

To review the scope of this specification, it is evident that I have disclosed the fundamental theory of the cable-tool drill and other analogous devices, pointing out that it is the destructive reactions occurring at the critical velocity or. resonance that have, up to the present, restricted the strokes per minute of such devices. Attention is directed to the economic advantages that will result from operation at higher speeds notably greatly increased capacity or drilling speed. Further, operation in the zone above the critical velocity will make it possible to operate at a speed which will result in any desired length of stroke I 4, for passing through the critical velocity with.

comparative safety, and three modifications have been disclosed of a means for varying the length of the surface stroke from zero to a maximum while the machine is in operation, which make the precision operation of the machine at such relatively high speeds safe and practical. Therefore, in its broader aspects, this invention involves a new and economically important method of operatingthe cable tool drill and other analogous devices.

What I claimas new and desire to secure by Letters Patent is:

1 The method of limiting the stresses in the elements of an elongated elastic metallic structure, vertically disposed in a hole, arranged to be reciprocated at its upper end by means having peak, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

2. The method of limiting the stresses in the elements of an elongated elastic metallic structure, vertically disposed in a hole, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent to peaks of vibrational resonance, comprising the steps of eliminating the amplitude of the reciprocation, while the frequency is below the peak, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

3. The method of limiting the stresses in the elements of an elongated elastic metallic structure, vertically disposed in a hole, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent peaks of vibrational resonance, comprising the steps of reducing the amplitude of the reciprocation, while operating at a frequency above the peak and then reducing the-frequency of the means for inducing the reciprocation to a point below the peak.

4. The method of limiting the stresses in the elements of an elongated elastic metallic structure, vertically disposed in a hole, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency,

.independently of the amplitude, while traversing 12 means for inducing the reciprocation to'a' poin below the peak.

5. The method of limiting the stresses in the elements of an elongated elastic metallic structure, vertically disposed in a hole, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency. independently of the amplitude, whiletraversin zones of destructivevibrational reactions adjacent peaks of vibrational resonance, comprising the steps of starting with reduced amplitude of reciprocation, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

6. The method of limiting the stresses in the elements of an elongated elastic metallic structure, vertically disposed in a hole, arranged to be reciprocated at its upper endby means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacents peaks of vibrational resonance, comprising the steps of starting with zero amplitude of reciprocation, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

7. The method of limiting the stresses in the elements of a sucker-rod pump, including a string of sucker-rods, arranged to be reciprocated at its upper end by means having a. variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent to peaks of vibrational resonance, comprising the steps of reducing the amplitude of the reciprocation, while the frequency is below the peak, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

8. The method of limiting the stresses in the elements of a sucker-rod pump, including a string of sucker-rods, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adiacent to peaks of vibrational resonance, comprising the steps of eliminating the amplitude of the reciprocation, while the frequency is .below the peak, increasing the frequency of the means for inducing the reciprocation to a-point above the peak and then increasing the amplitude of the reciprocation.

9. The method of limiting the stresses in the elements of a sucker-rod pump, including a string of sucker-rods, arranged to be reciprocated at its upper end by means having a variabl amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent peaks of vibrational resonance, comprising the steps of reducing the amplitude of the reciprocation, while operating at a frequency above the peak and then reducing the frequency of the means for inducing the reciprocation to a point below the peak.

10. The method of limiting the stresses in the elements of a sucker-rod pump, including a string a sucker-rods, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent peaks of vibrational resonance, comprising the steps of eliminating the amplitude of the reciprocation, while operating at a frequency above the peak and then reducing the frequency of the means for inducing the reciprocation to a point below the peak.

11. The method of limiting the stresses in the elements of a sucker-rod pump, including a string of sucker-rods, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adiacent peaks of vibrational resonance, comprising the steps of starting with reduced amplitude of reciprocation, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

12. The method of limiting the stresses in the elements of a sucker-rod pump. including a string of sucker-rods, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent peaks of vibrational resonance, comprising the steps of starting with zero amplitude of reciprocation, increasing the frequency of the means forinducingthe reciprocation to a point above the peak and then increasingthe amplitude of the reciprocation.

13. The method of limiting the stresses in the elements of a cable-tool drill, including a drillingtion to a point above the peak and then increasing the amplitude of the reciprocation.

14. The method of limiting the stresses in the elements of a cable-tool drill, including a drilling cable, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent to peaks oi' vibrational resonance, comprising the steps of eliminating the amplitude of the reciprocation, while the frequency is below the peak, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

15. The method of limiting the stresses in tire elements of a cable-tool. drill, including a drilling.

cable, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent peaks of vibrational resonance, comprising the steps of reducing the amplitude of the reciprocation, while'operating at a frequency above the peak and then reducing the frequency of the means for inducing the reciprocation to a point below the peak.

16. The method of limiting the stressesin the elements of a cable-tool drill, including a-drilling cable, arranged to be reciprocated at its upper end by means having a variable amplitude and a.

Number variable frequency, independently of the amD1i-- E-.-.

tude, while traversing zones of destructive vibrational reactions adjacent peaks of vibrational resonance, comprising the steps of eliminating the amplitude of the reciprocation, while operating at a frequency above the peak and then reducing the frequency of the means-for inducing the reciprocation to a point below the peak.

17. The method of limiting the stresse in the elements of a cable-tool drill, including a drilling cable, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibrational reactions adjacent peaks of vibrational resonance; comprising the steps of starting with reduced amplitude of reciprocation, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increasing the amplitude of the reciprocation.

18. The method of limiting the stresses in the elements of a cable-tool dril1, inc1uding a drilling cable, arranged to be reciprocated at its upper end by means having a variable amplitude and a variable frequency, independently of the amplitude, while traversing zones of destructive vibra-' tional reactions adjacent peaks of vibrational resonance, comprising the steps of starting with zero amplitude of reciprocation, increasing the frequency of the means for inducing the reciprocation to a point above the peak and then increas ing the amplitudeof the reciprocation.

, JOHN F. KENDRICK.

REFERENCES CITED The following references are of record in the file ofthis, patent:

UNITED STATES PATENTS Name Date Button Oct. 10, 1893 Rourke May 21, 1907 FOREIGN PATENTS Country Date Germany 1916 Germany Mar. 27, 1917 OTHER REFERENCES Timoshenko, Vibration- Problems in Engineering, 2nd ed., 1937, pp. 38-49.

Carstarphen, The CSM Magazine, pp. 15-30, Sept. 1931.

9Kendrick et al., Tras. ASME, vol. 123, pp. 15-31, 1 37.

Sprengling et al., Drilling Practice, DP. 64-72, 1940.

Kendrick, Oil Towards a Better Understanding of Cable Tool Drilling.

Slonneger, Production Practice, DD.-179-.188, 1937. a

Peterson, Petroleum Engineer, pp. 33-36, Feb. 1939.

Kendrick, Oil 8: Gas Journal, Dec. '13, 1947, Drilling Stresses Present in Cable-Tool Operations, Part1.

Number KendrlQk- Oil 8: Gas Journal, July 1, 1948, Drill- I ing Stresses Present in Cable-Tool p rations,

Part II.

Kendrick, on a Gas Journal, May 26, 1946,

pp. -172, 2nd ed., 1934'.

. Hartzell Oct. 14, 1924 a Gas Journal, Oct. '1, 194a, 

